This Application claims priority to Canadian Application Serial No. 2,346,517 filed May 4, 2001.
(Not applicable)
The present invention relates to a method to maintain isocapnia when breathing exceeds baseline breathing and a circuit therefor. Preferably, the circuit includes a non-rebreathing valve, a source of fresh gas, a fresh gas reservoir and a source of gas to be inhaled when minute ventilation exceeds fresh gas flow. Preferably the flow of the fresh gas is equal to minute ventilation minus anatomic dead space. Any additional inhaled gas exceeding fresh gas flow has a partial pressure of CO2 equal to the partial pressure of CO2 of arterial blood.
Venous blood returns to the heart from the muscles and organs partially depleted of oxygen (O2) and a full complement of carbon dioxide (CO2). Blood from various parts of the body is mixed in the heart (mixed venous blood) and pumped into the lungs via the pulmonary artery. In the lungs, the blood vessels break up into a net of small vessels surrounding tiny lung sacs (alveoli). The vessels surrounding the alveoli provide a large surface area for the exchange of gases by diffusion along their concentration gradients. After a breath of air is inhaled into the lungs, it dilutes the CO2 that remains in the alveoli at the end of exhalation. A concentration gradient is then established between the partial pressure of CO2 (PCO2) in the mixed venous blood (PvCO2) arriving at the alveoli and the alveolar PCO2. The CO2 diffuses into the alveoli from the mixed venous blood from the beginning of inspiration (at which time the concentration gradient for CO2 is established) until an equilibrium is reached between the PCO2 in blood from the pulmonary artery and the PCO2 in the alveolae at some time during breath. The blood then returns to the heart via the pulmonary veins and is pumped into the arterial system by the left ventricle of the heart. The PCO2 in the arterial blood, termed arterial PCO2 (PaCO2) is then the same as was in equilibrium with the alveoli. When the subject exhales, the end of his exhalation is considered to have come from the alveoli and thus reflects the equilibrium CO2 concentration between the capillaries and the alveoli. The PCO2 in this gas is the end-tidal PCO2 (PETCO2). The arterial blood also has a PCO2 equal to the PCO2 at equilibrium between the capillaries and alveoli.
With each exhaled breath some CO2 is eliminated and with each inhalation, fresh air containing no CO2 is inhaled and dilutes the residual equilibrated alveolar PCO2, establishing a new gradient for CO2 to diffuse out of the mixed venous blood into the alveoli. The rate of breathing, or ventilation (VE), usually expressed in L/min, is exactly that required to eliminate the CO2 brought to the lungs and establish an equilibrium PETCO2 and PaCO2 of approximately 40 mmHg (in normal humans). When one produces more CO2 (e.g. as a result of fever or exercise), more CO2 is carried to the lungs and one then has to breathe harder to wash out the extra CO2 from the alveoli, and thus maintain the same equilibrium PaCO2. But if the CO2 production stays normal, and one hyperventilates, then excess CO2 is washed out of the alveoli and the PaCO2 falls.
It is important to note that not all VE contributes to elimination of CO2. The explanation for this is with reference to the schematic in the lung depicted in FIG. 10. The lung contains two regions that do not participate in gas equilibration with the blood. The first comprises the set of conducting airways (trachea and bronchi) 100 that act as pipes directing the gas to gas exchanging areas. As these conducting airways do not participate in gas exchange they are termed anatomic dead space 102 and the portion of VE ventilating the anatomic dead space is termed anatomic dead space ventilation (VDan). The same volume of inhaled gas resides in the anatomic dead space on each breath. The first gas that is exhaled comes from the anatomic dead space and thus did not undergo gas exchange and therefore will have a gas composition similar to the inhaled gas. The second area where there is no equilibration with the blood comprises the set of alveoli 103 that have lost their blood supply; they are termed alveolar dead space 104. The portion of VE ventilating the alveolar dead space is termed alveolar dead space ventilation (VDalv). Gas is distributed to alveolar dead space in proportion to their number relative to that of normal alveoli (normal alveoli being those that have blood vessels and participate in gas exchange with blood). That portion of VE that goes to well perfused alveoli and participates in gas exchange is called the alveolar ventilation (VA). In FIG. 10, the numeral references 105 and 106 indicate the pulmonary capillary and the red blood cell, respectively.
Prior art circuits used to prevent decrease in PCO2 resulting from increased ventilation, by means of rebreathing of previously exhaled gas are described according to the location of the fresh gas inlet, reservoir and pressure relief valve with respect to the patient. They have been classified by Mapleson and are described in Dorsch and Dorsch pg 168.
Mapleson A
The circuit comprises a pressure relief valve nearest to the patient, a tubular reservoir and fresh inlet distal to the patient. In this circuit, on expiration, dead space gas is retained in the circuit, and after the reservoir becomes full, alveolar gas is lost through the relief valve. Dead space gas is therefore preferentially rebreathed. Dead space gas has a PCO2 much less than PaCO2. This is less effective in maintaining PCO2 than rebreathing alveolar gas, as occurs with the circuit of the present invention.
Mapleson B and C
The circuit includes a relief valve nearest the patient, and a reservoir with a fresh gas inlet at the near patient port. As with Mapleson A dead space gas is preferentially rebreathed when minute ventilation exceeds fresh gas flow. In addition, if minute ventilation is temporarily less than fresh gas flow, fresh gas is lost from the circuit due to the proximity of the fresh gas inlet to the relief valve. Under these conditions, when ventilation once again increases, there is no compensation for transient decrease in ventilation as the loss of fresh gas will prevent a compensatory decrease in PCO2.
Mapleson D and E
Mapleson D consists of a circuit where fresh gas flow enters near the patient port, and gas exits from a pressure relief valve separated from the patient port by a length of reservoir tubing. Mapleson E is similar except it has no pressure relief valve allowing the gas to simply exit from an opening in the reservoir tubing. In both circuits, fresh gas is lost without being first breathed. The volume of gas lost without being breathed at a given fresh flow is dependent on the pattern of breathing and the total minute ventilation. Thus the alveolar ventilation and the PCO2 level are also dependent on the pattern of breathing and minute ventilation. Fresh gas is lost because during expiration, fresh gas mixes with expired gas and escapes with it from the exit port of the circuit. With the present invention, all of the fresh gas is breathed by the subject.
There are many different possible configurations of fresh gas inlet, relief valve, reservoir bag and CO2 absorber (see Dorsch and Dorsch, pg. 205-207). In all configurations, a mixture of expired gases enters the reservoir bag, and therefore rebreathed gas consists of combined dead space gas and alveolar gas. This is less efficient in maintaining PCO2 constant than rebreathing alveolar gas preferentially as occurs with our circuit, especially at small increments of V above the fresh gas flow.
The present invention comprises a method and a circuit that maintains a constant PCO2 More particularly, the present invention maintains a constant PCO2 by:
1) setting FGF equal to the baseline minute ventilation less the anatomical dead space ventilation (VDan); and
2) establishing PrgCO2 being equal to the PaCO2 rather than the PvCO2 to increase accuracy of the methods herein disclosed.
In the present invention, when minute ventilation is temporarily less than fresh gas flow, no fresh gas is lost from the circuit. Instead, the reservoir acts as a buffer to store extra fresh gas. When ventilation increases once more, the subject breathing the accumulated fresh gas allows PCO2 to return to the previous level.
A circuit to maintain isocapnia is also provided by the invention. The circuit includes a non rebreathing valve, a source of fresh gas, a fresh gas reservoir and a source of gas to be inhaled when minute ventilation exceeds fresh gas flow. Preferably, the flow of fresh gas is equal to minute ventilation minus anatomic dead space. Any additional inhaled gas exceeding fresh gas flow has a partial pressure of CO2 equal to the partial pressure of CO2 of arterial blood.
The invention further provides a method of measuring anatomical and/or alveolar dead space ventilation by using a breathing circuit consisting of a non rebreathing valve, a source of fresh gas, a fresh gas reservoir and a source of gas with a partial pressure of CO2 substantially equal to that of arterial blood.
In one embodiment of the invention, the non-rebreathing circuit comprises an exit port, a non-rebreathing valve, a source of fresh gas, a fresh gas reservoir, and a reservoir gas supply. From the exit port, gases are supplied from the circuit to the patient. The non-rebreathing valve has a one-way valve permitting gases to be delivered to the exit port to the patient, but prevents gases from passing into the circuit. The source of fresh gas may be oxygen, air or the like excluding CO2 (air containing physiologically insignificant amount of CO2) and is in communication with the non-rebreathing valve to be delivered to the patient. The fresh gas reservoir is in communication with the source of fresh gas flow for receiving excess fresh gas not breathed by the patient from the source of fresh gas and for storage thereof, wherein as the patient breathes gas from the source of fresh gas flow and from the fresh gas reservoir are available depending on the minute ventilation level. The reserve gas supply contains CO2 and other gases (usually oxygen) preferably having a partial pressure of the CO2 approximately equal to the partial pressure of CO2 in the arterial blood of the patient. The reserve gas supply is delivered to the non-rebreathing valve to make up that amount of gas required by the patient for breathing that is not fulfilled from the gases delivered from the source of fresh gas flow and the fresh gas reservoir. The source of gas, the fresh gas reservoir and the reserve gas supply are disposed on the side of the non-breathing valve remote from the exit port.
Preferably, a pressure relief valve is provided in the circuit in communication with the fresh gas reservoir in the event that the fresh gas reservoir overfills with gas so that the fresh gas reservoir does not break, rupture or become damaged in any way.
The reserve gas supply preferably includes a demand valve regulator. When additional gas is required, the demand valve regulator opens the communication of the reserve gas supply to the non-rebreathing valve for delivery of the gas thereto. When additional gas is not required, the demand valve regulator is closed and only fresh gas flows from the source of fresh gas and from the fresh gas reservoir to the non-rebreathing valve. The source of fresh gas is set to supply fresh gas (non-CO2-containing gas) at a rate equal to desired alveolar ventilation for the elimination of CO2, that is, the baseline minute ventilation minus anatomical dead space.
The basic concept of the present invention is when breathing increases, flow of fresh gas (inspired PCO2=0) from the fresh gas flow contributing to elimination of CO2 is kept constant, and equal to the baseline minute ventilation minus anatomical dead space. The remainder of the gas inhaled by the subject (from the reserve gas supply) has a PCO2 equal to that of arterial blood, resulting in the alveolar PCO2 stabilizing at the arterial PCO2 level regardless of the level of ventilation as long as minute ventilation minus anatomical dead space is greater than the fresh gas flow. In the event that the desired PaCO2 is a particular value, which may be higher or lower than the initial PaCO2 of the subject, then the PCO2 having an adjustable feature of the reserve gas may simply be set equal to the desired PaCO2. If the PaCO2 is specifically desired to remain equal to the initial PaCO2 of the subject, then the PaCO2 can be measured by obtaining a sample of arterial blood from any artery, and the PCO2 of the reserve gas set equal to this valve. Alternatively, an estimation of the PaCO2 can be made from PETCO2. PETCO2 is determined by measuring the PCO2 of expired breath using a capnograph usually present or easily available in medical and research facilities to persons skilled in the art.
In effect, the present invention passively causes the amount of CO2 breathed in by the patient to be proportional to the amount of total breathing, thereby preventing any perturbation of the arterial PCO2. This is unlike prior art servo-controllers which always attempt to compensate for changes. Persons skilled in the art, however, may choose to automate the circuit by using a servo-controller or computer to monitor minute ventilation levels and deliver inspired gas with the concentrations of CO2 substantially equal to that of those from fresh gas and reserve gas were the gases mixed together.
The non-rebreathing circuit provided by the present invention can also be used to enable a patient to recover more quickly from, and to hasten the recovery of the patient after vapor anaesthetic administration, or poisoning with carbon monoxide, methanol, ethanol, or other volatile hydrocarbons.
According to another aspect of the invention, a method of treatment of an animal or person is provided. The method comprises delivering to a patient gases which do not contain CO2 at a specific rate, and gases containing CO2 to maintain the same PCO2 in the patient, at the rate of ventilation of the patient which exceeds the rate of administration of the gases which do not contain CO2 independent of the rate of ventilation.
The circuit and method of treatment can also be used for any circumstance where it is desirable to dissociate the minute ventilation from elimination of carbon dioxide such as respiratory muscle training, investigation of the role of pulmonary stretch receptors, tracheobronchial tone, expand the lung to prevent atelectasis, exercise, and control of respiration and other uses as would be understood by those skilled in the art.
The circuit and method of treatment of the present invention may also be used by deep sea divers and astronauts to eliminate nitrogen from the body. It can also be used to treat carbon monoxide poisoning under norma baric or hyper baric conditions. In this case, the fresh gas would contain a higher concentration of oxygen than ambient air, for example, 100% O2, and the reserve gas will contain approximately 5.6% CO2 and a high concentration of oxygen, for example, 94% Of O2.
In another embodiment of the invention, a method of controlling PCO2 in a patient at a predetermined desired level is provided comprising a breathing circuit which is capable of organizing exhaled gas so as to be preferentially inhaled during re-breathing when necessary by providing alveolar gas for re-breathing in preference to dead space gas. The preferred circuit in effecting this method includes a breathing port for inhaling and exhaling gas, a bifurcated conduit adjacent to the port. The bifurcated conduit has a first and a second conduit branches. The first conduit has a fresh gas inlet and a check valve allowing the passage of inhaled fresh gas to the port but closing during exhalation. The second conduit branch includes a check valve which allows passage of exhaled gas through the check valve but prevents flow back to the port. A fresh gas reservoir is located at the terminus of the first conduit branch, while an exhaled gas reservoir is located at the terminus of the second conduit branch. An interconnecting conduit having a check valve therein is located between the first and the second conduit branches to result in the fresh flow gas in the circuit equal to baseline minute ventilation minus ventilation of anatomic dead space for the patient. In the exhaled gas reservoir, the exhaled gas is preferably disposed nearest the open end thereof, and the alveolar gas is located proximate the end of the reservoir nearest the terminus of the second conduit branch, so that the shortfall differential of PCO2 is made of alveolar gas being preferentially rebreathed, thereby preventing a change in the PCO2 level of alveolar gas despite the increased minute ventilation.
It is important to set up the fresh gas flow to be baseline minute ventilation minus anatomic dead space ventilation. In this way, once it is desired to increase the minute ventilation, a slight negative pressure will exist in the interconnecting conduit during inhalation, opening its check valve and allowing further breathing beyond the normal level of ventilation to be supplied by previously exhaled gas.
The present invention also provides a method of enhancing the results of a diagnostic procedure or medical treatment. A circuit which is capable of organizing exhaled gas so as to provide to the patient preferential rebreathing of alveolar gas in preference to dead space gas is provided. The patient is ventilated when a rate greater than the fresh gas flow is desired, and when hypercapnia is desired to induce. The fresh gas flow is passively decreased to provide a corresponding increase in rebreathed gas. The hypercapnia is continuously induced until the diagnostic or medical procedure is complete. Examples of the medical procedure includes MRI, radiation treatment or the like.
The present invention can also be applied to treat or assist a patient, preferably human, during a traumatic event characterized by hyperventilation. A breathing circuit in which alveolar ventilation is equal to the fresh gas flow and increases in alveolar ventilation with increases in minute ventilation is prevented, is provided. The circuit is capable of organizing exhaled gas provided to the patient and preferential rebreathing alveolar gas in preference to dead space gas following ventilating the patient at a rate of normal minute ventilation, preferably approximately 5 L per minute. When desired, hypercapnia is induced to increase arterial PCO2 and prevent the PCO2 level of arterial blood from dropping. The normocapnia is maintained despite the ventilation is increased until the traumatic hyperventilation is complete. As a result, the effects of hyperventilation experienced during the traumatic event are minimized. This can be applied when a mother is in labor and becomes light headed or when the baby during the delivery is effected with the oxygen delivery to its brain being decreased as a result of contraction of the blood vessels in the placenta and fetal brain. A list of circumstances in which the method enhancing the diagnostic procedure results or the experience of the traumatic even are listed below.
Applications of the method and circuit includes:
1) Maintenance of constant PCO2 and inducing changes in PCO2 during MRI.
2) Inducing and/or marinating increased PCO2:
a) to prevent or treat shivering and tremors during labor, post-anesthesia, hypothermia, and certain other pathological states;
b) to treat fetal distress due to asphyxia;
c) to induce cerebral vasodilatation, prevent cerebral vasospasm, and provide cerebral protection following subarachnoid hemorrhage cerebral trauma and other pathological states;
d) to increase tissue perfusion in tissues containing cancerous cells to increase their sensitivity to ionizing radiation and delivery of chemotherapeutic agents;
e) to aid in radio diagnostic procedures by providing contrast between tissues with normal and abnormal vascular response; and
f) protection of various organs such as the lung, kidney and brain during states of multi-organ failure.
3) Prevention of hypocapnia with O2 therapy, especially in pregnant patients.
4) Other applications where O2 therapy is desired and it is important to prevent the accompanying drop in PCO2.
By carrying out the above method and preferably with the above circuit, an improved method of creating MRI images is disclosed to maintain a constant PCO2 and induce changes in that PCO2 level during the MRI procedure in order to facilitate improvement in the quality of the images being obtained. The prior art Mapleson D and E circuits predictably may work with the method of the present invention as well as a standard circuit with the carbon dioxide filter bypassed or removed; however fresh gas will be wasted and the efficiency would be reduced.
A method of delivering to a patient, preferably human, inhaled drugs such as gases, vapors or suspensions of solid particles, particles or droplets, for example, nitric oxide, anesthetic vapors, bronchodilators or the like, using the above circuit to increase the efficiency of delivery allows the quantification of the exact dose.
A method of delivering to a patient, preferably human, pure oxygen is provided. The circuit described above increases the efficiency of delivery because all the fresh gas is inhaled by the patient, or to deliver the oxygen to the patient in a more predictable way, allowing the delivery of a precise concentration of oxygen.
When minute ventilation minus anatomical dead space ventilation is greater than or equal to fresh gas flow, the above circuit prevents loss of fresh gas and ensures that the patient receives all the fresh gas independent of the pattern of breathing since fresh gas alone enters the fresh gas reservoir, and exhaled gas enters its own separate reservoir. The fresh gas reservoir bag is large enough to store fresh gas for 5-10 seconds or more of reduced ventilation or total apnea, ensuring that even under these circumstances fresh gas will not be lost. The preferred circuit prevents rebreathing at a minute ventilation equal to the fresh gas flow because the check valve in the interconnecting conduit does not open to allow rebreathing of previously exhaled gas unless a negative pressure exist on the inspiratory side of the conduit of the circuit. Also, when minute ventilation exceeds the fresh gas flow, a negative pressure occurs in the inspiratory conduit, opening the conduit""s check valve. The circuit provides that after the check valve opens, alveolar gas is rebreathed in preference to dead space gas because the interconnecting conduit is located such that exhaled alveolar gas will be closest to it and dead space gas will be further from it. When the fresh gas flow is equal to VExe2x88x92VDan, the volume of rebreathed gas will ventilate the anatomical dead space only, leaving the alveolar ventilation unchanged. The exhaled gas reservoir is preferably sized at 3 L which is well in excess of the volume of an individual""s breath, therefore it is unlikely that the patient shall be able to breathe any room air entering via the opening at the end of the exhaled gas reservoir.
The basic approach of preventing a decrease in PCO2 with increased ventilation is similar to that of the non-rebreathing system. In brief, only the fresh gas contributes to alveolar ventilation (VA) which establishes the gradient for CO2 elimination. All gas breathed in excess of the fresh gas entering the circuit, or the fresh gas flow, is rebreathed gas. The terminal part of the exhaled gas contains gas that has been in equilibrium with arterial blood and hence has a PCO2 substantially equal to arterial blood. The Fisher (WO98/41266) patent teaches that the closer PCO2 in the inhaled gas to PvCO2, the less the effect on CO2 elimination. Yet, it would not maintain a constant PaCO2 as VE increases. The present invention discloses that the greater the ventilation of gas with a PCO2 equal to PvCO2, the closer the PaCO2 gets to PvCO2. The present invention also discloses that when PCO2 of inhaled gas is substantially equal to PaCO2, increased ventilation will not tend to change the PaCO2. Since the terminal part of the exhaled gas contains gas that has been in equilibrium with arterial blood and hence has a PCO2 substantially equal to arterial blood, the PaCO2 will be unchanged regardless of the extent of rebreathing.
With the use of the circuit of the present invention:
1. All of the fresh gas is inhaled by the subject when minute ventilation minus anatomical dead space is equal to or exceeds fresh gas flow.
2. The xe2x80x9calveolar gasxe2x80x9d is preferentially rebreathed when minute ventilation minus anatomical dead space exceeds the fresh gas flow.
3. When minute ventilation minus anatomical dead space is equal to or greater than fresh gas flow, all the fresh gas contributes to alveolar ventilation.
In another embodiment of the invention, a method of establishing a constant flow of fresh gas in the form of atmospheric air forced as a result of breathing efforts by the patient, but independent of the extent of ventilation, is provided. The flow is delivered into a breathing circuit such as that taught by Fisher et al., (non-rebreathing) designed to keep the PCO2 constant by providing expired gas to be inhaled when the minute ventilation exceeds the flow of fresh gas. Furthermore, there is provided a compact expired gas reservoir capable of organizing exhaled gas so as to be preferentially inhaled during rebreathing when necessary by providing alveolar gas for re-breathing in preference to dead space gas. The preferred circuit in effecting the above-mentioned method includes a breathing port for inhaling and exhaling gas, a bifurcated conduit adjacent to the port in substantially a Y-shape. The bifurcated conduit has a first and a second conduit branches. The first conduit has an atmospheric air inlet the flow through which is controlled by a resistance for example that being provided by a length of tubing, and a check valve allowing the passage of inhaled atmospheric air to the port but closing during exhalation. The second conduit branch includes a check valve which allows passage of exhaled gas through the check valve but prevents flow back to the port. An atmospheric air aspirator (AAA) is located at the terminus of the first conduit branch, while an exhaled gas reservoir of about 3 L in capacity is located at the terminus of the second conduit branch. The AAA comprises a collapsible container tending to recoil to open position. An interconnecting conduit having a check valve therein is located between the first and the second conduit branches. When minute ventilation minus anatomic dead space ventilation is equal to the rate of atmospheric air aspirated into the circuit, for example, 4 L per minute, atmospheric air enters the breathing port from the first conduit branch at a predetermined rate and preferably 4 L per minute. Meanwhile, the exhaled gas at a rate of 4 L per minute travels town to the exhaled gas reservoir. When it is desirable for the minute ventilation to exceed the fresh gas flow, for example, 4 L per minute, the patient will inhale expired gas retained in the expired gas reservoir through the interconnecting conduit at a rate making up the shortfall of the atmospheric air.
While setting the fresh gas flow to maintain a desired PCO2, it is important to set up the atmospheric air aspirator be allowed to first be depleted of gas until it just empties at the end of the inhalation cycle. In this way, once it is desired to increase the minute ventilation, the increased breathing effort required to do so will further decrease the sub-atmospheric pressure in the first conduit branch, being the inspiratory limb, and open the check valve in the interconnecting conduit to allow further breathing of gas beyond the level of ventilation supplied by the volume of atmospheric air aspirated into the circuit during the entire breathing cycle.
The circuit of the present invention is particularly applicable when atmospheric air is a suitable form of fresh gas and when it is inconvenient or impossible to access a source of compressed gas or air pump to provide the fresh gas flow. During mountain climbing or working at high altitude, some people tend to increase their minute ventilation to an extent greater than that required to optimize the alveolar oxygen concentration. This will result in an excessive decrease in PCO2 which will in turn result in an excessive decrease in flood flow and hence oxygen delivery to the brain. By using the above circuit at high altitude a limit can be put on the extent of decrease in PCO2 and thus maintain the oxygen delivery to the brain in the optimal range.
During resuscitation of an asphyxiated newborn or an adult suffering a cardiac arrest, the blood flow through the lungs is remarkably slow during resuscitation attempts. Even normal rates of ventilation may result an excessive elimination of CO2 from the blood. As the blood reaches brain, the low PCO2 may constrict the blood vessels and limit the potential blood flow to the ischemic brain. By attaching the isocapnia circuit provided by the invention to the gas inlet port of a resuscitation bag and diverting all expiratory gas to the expiratory gas reservoir bag, the decrease of PCO2 would be limited.
The isocapnia circuit of the present invention can be applied to enhance the results of a diagnostic procedure or a medical treatment by providing a circuit without a source of forced gas flow and being capable of organizing exhaled gas. With the circuit, preferential rebreathing of alveolar gas in preference to dead space gas is provided when the patient is ventilating at a rate greater than the rate of atmospheric air aspirated, and when inducing hypercapnia is desired. By decreasing the rate of aspirated atmospheric air, a corresponding increase in rebreathed gas is passively provided to prevent the PCO2 level of arterial blood from dropping despite increase in minute ventilation. The step of inducing hypercapnia is continued until the diagnostic or medical therapeutic procedure is complete. The results of the diagnostic or medical procedure are thus enhanced by carrying out the method in relation to the results of the procedure had the method not been carried out. Examples of such procedures include MRI or preventing spasm of brain vessels after brain hemorrhage, radiation treatments or the like.
The present invention can also be applied to treat or assist a patient, preferably human, during a traumatic event characterized by hyperventilation. A circuit that does not require a source of forced gas flow, in which alveolar ventilation is equal to the rate of atmospheric air aspirated and increases in alveolar ventilation with increases in minute ventilation is prevented, is provided. For example, the isocapnia circuit as described above, is capable of organizing exhaled gas provided to the patient preferential rebreathing alveolar gas in preference to dead space gas following ventilating the patient at a rate of normal minute ventilation, preferably approximately 5 L per minute. When desired, hypercapnia is induced to increase arterial PCO2 and prevent the PCO2 level of arterial blood from dropping. The normocapnia is maintained despite the ventilation being increased until the traumatic hyperventilation is complete. As a result, the effects of hyperventilation experienced during the traumatic event are minimized. This can be applied when a mother is in labor and becomes light headed or the baby during the delivery is effected with the oxygen delivery to its brain being decreased as a result of contraction of the blood vessels in the placenta and fetal brain. A list of circumstances in which the method enhancing the diagnostic procedure results or the experience of the traumatic even are listed below.
Applications of the method and circuit includes:
1) Maintenance of constant PCO2 and inducing changes in PCO2 during MRI.
2) Inducing and/or marinating increased PCO2:
a) to prevent or treat shivering and tremors during labor, post-anesthesia, hypothermia, and certain other pathological states;
b) to treat fetal distress due to asphyxia;
c) to induce cerebral vasodilatation, prevent cerebral vasospasm, and provide cerebral protection following subarachnoid hemorrhage cerebral trauma and other pathological states;
d) to increase tissue perfusion in tissues containing cancerous cells to increase their sensitivity to ionizing radiation and delivery of chemotherapeutic agents;
e) to aid in radio diagnostic procedures by providing contrast between tissues with normal and abnormal vascular response; and
f) protection of various organs such as the lung, kidney and brain during states of multi-organ failure.
3) Prevention of hypocapnia with O2 therapy, especially in pregnant patients.
4) Other applications where O2 therapy is desired and it is important to prevent the accompanying drop in PCO2.
When minute ventilation is greater than or equal to the rate of atmospheric air aspirated, the above-mentioned preferred circuit ensures that the patient receives all the atmospheric air aspirated into the circuit, independent of the pattern of breathing; since atmospheric air alone enters the fresh gas reservoir and exhaled gas enters its own separate reservoir and all the aspirated air is delivered to the patient during inhalation before rebreathed exhaled gas. The atmospheric air aspirator preferably large enough not to fill to capacity during prolonged exhalation, when the total minute ventilation exceeds the rate of atmospheric air aspiration ensuring that under these circumstances atmospheric air continues to enter the circuit uninterrupted during exhalation. The preferred circuit prevents rebreathing at a minute ventilation equal to the rate of air being aspirated into the atmospheric air aspirator because the check valve in the interconnecting conduit does not open to allow rebreathing of previously exhaled gas unless a sub-atmospheric pressure less than that generated by the recoil of the aspirator exists on the inspiratory side of the conduit of the circuit. The circuit provides that after the check valve opens, alveolar gas is rebreathed in preference to dead space gas because the interconnecting conduit is located such that exhaled alveolar gas contained in the tube conducting the expired gas into the expiratory reservoir bag will be closest to it and dead space gas will be mixed with other exhaled gases in the reservoir bag. In the preferred embodiment, the exhaled gas reservoir is preferably sized at about 3 L which is well excess of the volume of an individuals breath. When the patient inhales gas from the reservoir bag, the reservoir bag collapses to displace the volume of gas extracted from the bag, minimizing the volume of atmospheric air entering the bag.
The basic approach of the present invention to prevent a decrease in PCO2 with increase ventilation is to arrange that the fresh gas enters, the circuit at a rate equal to the desired minute ventilation minus anatomic dead space ventilation. In brief, breathing only fresh gas contributes to alveolar ventilation (VA) which establishes the gradient for CO2 elimination. All gas breathed in excess of the fresh gas entering the circuit, or the fresh gas flow, is rebreathed gas. The closer the partial pressure of carbon dioxide in the inhaled gas to that of arterial blood, the less the effect on CO2 elimination. With increased levels of ventilation, greater volumes of previously exhaled gas are breathed. The rebreathed gas has a PCO2 substantially equal to that of arterial blood, thus contributing little if anything to alveolar ventilation, and allowing the PETCO2 and PaCO2 to change little.
Further, if the fresh gas flow is equal to the minute ventilation minus the anatomic dead space ventilation, when minute ventilation is equal to or exceeds the rate of atmospheric air aspirated into the circuit, then all of the delivered fresh gas remains constant and equal to the resting alveolar ventilation. The xe2x80x9calveolar gasxe2x80x9d is preferentially rebreathed when minute ventilation exceeds the fresh gas flow. These, as well as other features of the present invention will become more evident upon reference to the drawings and detailed description of the invention.